1. Field of the Invention
The invention relates to a semiconductor device and more particularly to a semiconductor device with improved electrical characteristics and a method for fabricating thereof.
2. Description of the Related Art
With the wide application of integrated circuits (ICs), several kinds of semiconductor devices with higher efficiencies and lower costs have been produced based on different objectives. The memory device, such as a dynamic random access memory (DRAM), is such an important semiconductor device in the electronics industry. Typically, one DRAM cell includes one transistor and one capacitor. In order to increase the memory device integration, it is required to shrink the size of the memory cell and the transistor so as to manufacture the DRAM with higher memory capacity and higher processing speed.
A traditional DRAM with a plane transistor covers large areas of the semiconductor substrate and cannot satisfy the demand of high integration. Therefore, a vertical transistor which can save space is a trend in the fabrication of a memory cell.
FIG. 1 is a cross section of a conventional vertical transistor for a memory cell with a buried word line (WL). The transistor 10 includes a semiconductor substrate 102 having an opening 100a therein. Doping regions 102 are formed on both sides of the opening 100a to serve as source/drain regions of the transistor 10. A gate electrode 104 is disposed in the bottom of the opening 100a and is referred to as a buried WL. A gate dielectric layer 106 is interposed between the opening 100a and the gate electrode 104. A cap oxide 108 is formed in the opening 100a to cover the gate electrode 104.
However, the top corners C of the gate electrode 104 adjacent to the doping regions 102 may induce electric field concentration, resulting in an enhancement of gate induced drain leakage (GIDL). The increased GIDL is not desired because it reduces the reliability of the transistor 10.
Accordingly, there is a need to develop a novel semiconductor device which is capable of mitigating or eliminating the aforementioned problem.